There is an organisation called the Long Now Foundation (see link) which is planning on constructing a clock which will last for 10,000 years, called the Clock of the Long Now. The idea of this is to encourage people to think in the long term. Their version is an actual mechanical clock. This idea is
a much simpler method of doing something similar.

It uses a large slab of rock (I like granite myself but I'm open to suggestions) with a little stream of water running across the top. Over the centuries, the water would wear away the rock, slicing a thin channel through it, kind of like a very slow version of cutting cheese with a wire. The date would be marked at various points down the front of the rock, so you could just look at how far down the channel had been cut to find out the year.

The stream of water would have to remain constant so that it would cut at a predictable rate. This could be arranged by having a large reservoir, which flowed into a bowl that was always overflowing, and so always at the same level. Then the stream could come out of a hole at the bottom of the bowl and would always be constant. The reservoir would be replenished by rainfall. The whole thing could be underground to avoid getting clogged up with dirt. (Under permeable rock of course so that the rain could reach the reservoir)

I think this would do basically the same job as a mechanical clock, and it would have two advantages. Firstly, it's a lot simpler so it would be more reliable. And secondly, it would be pretty cheap to set up, so you could build them all over the place. Every city could have a Rock of the Long Now. This might do a better job of encouraging people to look at the long term than if there was just one of them stuck out in a desert somewhere.

The Long Now have spent years mulling
over the exact nature of the 'clock' - the
concept of a mechanical clock was not
presumed and rocks, water, light etc
were discussed at great length,
though they did come up with a rather
Jules Verne brass and wheels affair,
which I thought was marvelous.

Brian Eno has been inspired to design
the bell chimes for the clock, so that
each chime is unique, so hearing one
would convey a precise time. He was
helped by new software that accurately
conveyed the sounds of bells that are
repeatedly struck.

So let me get this right. The whole thing, resevoir included, is installed underground, under permeable rock. I'm trying desperately to mate that with the part about it being simple, cheap to set up, and you could build them all over the place.

Interesting, but who's going to keep them up (you know, weed out moss, mark the dates, that sort of thing). You'd have to start up some sort of Masonic/ Order of the Rose Cross type cult.
Although that could be fun.
' My Brother, to begin your initiation into the Order of the Long Now, I place in your right hand the Sacred Chisel, used since time immemorial to mark the Sacred Rock of the Long Now... Now run along and practice on that brick; we don't want you screwing this up for posterity...'
Anyhoo, my point is you can't rely on the erosion rate (and thus the date markings) to remain constant over the centuries, so what are you going to do, *spacemoggy* ?(if that *is* your real name) What are you going to do???????

[spacemoggy] problem. A rocktype is an aggregate of minerals - it is not uniform. Granite is comprised of mica, plagioclase or orthoclase and finally, quartz. It'll also have smatterings of zircons and other minerals.

The point is simply that the rock is not uniform thus the water will cut through it at different rates depending on the mineralogy at that particular point.

[The Kat] What this is about is simply marking the passage of time, in terms of decades rather than in terms of hours or weeks. I thought of this when I went to a monastery where there was a stone altar that had had a trickle of water running over it for several hundred years, and it had worn away the stone, and I thought it would be nice to have a place particularly set up for that so you could see it change over the course of your life.

[Bluetirtle] the dates are going to be marked on when it's built, because the whole point of it is that the erosion rate is constant and therefore predictable. Which brings us to...

[Everyone who said the erosion rate won't be constant] I can see what you mean, especially JonTheGeologist's comment (I was hoping to hear from you on this) about the lack of uniformity in rock. Is that true of all rocks? There must be some rocks which are highly uniform. Jon mentions a 'monomineral' rock - what would be an example of this? If all else fails, you could make it of a man-made 'rock', like concrete.

[Whatastrangeperson] Please read what I wrote again, paying particular attention to the part where it says the reservoir would be replenished by rainfall. ;-)

And finally....
[Zigness] Thank you. I will take that as a compliment.

If it was hooked up to a resevoir, the rate of erosion would very, very slowly start to deccelerate, as there would already be solute material inside the water. It would be more consistent if it was plugged into a piping system, though this would be impractical

Hey, chill out! Obviously s'moggy's going for longevity here, not accuracy. Would it be marketed as a Rock Clock, or a Clock Rock? Lousy for timing Olympic events, but come the next Ice Age, hey, we're covered! Can hardly wait for the wrist or pocket version (can you say soggy pockets?) <hey, my wristwatchrock is very dependable. It's right twice per millenium!>

Another way might be to use a cheese cutter through glass, with a big weight. Glass behaves like liquid over a very long period of time. Stained glass windows in old churches have glass which is thicker at the bottom.

The problem here is that chemical erosion will predominate, so the clock will be thrown out of kilter by dissolved pollutantsespecially sulfur compounds. With volcanoes or nearby coal burning plants, your clock could run thousands of years fast.

The chemical erosion could be a show-stopper, because obviously you can't predict what the future chemical compostion of the rainwater will be. This idea was based on physical erosion predominating, but I don't know enough about rocks to know if thats possible. If chemical erosion always predominates then the idea won't work.